1. Field of the Invention
This invention relates to the use of polarized light to detect small particles. More specifically, this invention relates to a means for detection of non-spherical particles in a process chamber used for the manufacture of integrated circuits.
2. Discussion of the Related Art
Particle detection is widely used in vacuum process equipment, such as that involved in the processing of semiconductor wafers, because even a small number of particles in the manufacturing process can lead to substantial yield loss.
Most particle detectors or monitors are designed based on a dark field technique. Several examples of particle monitors using a dark field technique are disclosed in U.S. Pat. No. 4,739,177 to Peter Borden, entitled "Particle Detector for Wafer Processing Equipment," filed on Sep. 16, 1986 and issued on Jun. 19, 1988. In the dark field technique, a laser beam is projected through a region where particles are expected to pass, and photodetectors or photocells are placed off-axis near the laser beam to detect the light the particles scatter from the laser beam. In a dark field particle detector, the laser beam is not incident on the photodetector. (Hence, the term "dark field" technique.) The scattered light detected by the off-axis photodetector is converted to an electrical pulse that indicates the presence of the particle.
However, a dark field particle detector has numerous inherent limitations. In particular, such a particle detector is very sensitive to background light or noise. For example, when used as a particle detector in a sputtering process which uses plasma, light from plasma glows, or from dirt present on the optics, can scatter light from the laser beam to the photodetectors. Also, since the photodetectors of a dark field particle detector must be placed in close proximity to the laser beam, such a particle detector is inherently limited in where it can be deployed. In particular, such a dark field particle detector cannot be readily placed inside a processing chamber where the semiconductor wafers are being processed.
A bright field particle detector overcomes some of the difficulties encountered in the use of a dark field particle detector. In a "bright field" sensor or detector, a laser beam is shone directly on the sensing photodetector. Particles passing through the laser beam scatter light from the laser beam, thereby reducing its intensity and, consequently, reducing the photocell current when the laser beam impinges the photocell. Since a bright field particle detector does not require the photocells to be placed in close proximity to the laser beam along its path, the bright field technique allows the laser beam to be shone across processing chambers. Further, the bright field technique is inherently less sensitive to background light or noise, since the bright field detector receives only the input stimulus from a small angular aperture, which corresponds to the size of the laser beam.
Bright field sensors, however, are susceptible to shot noise. Thus, bright field sensors are traditionally regarded as lacking the requisite sensitivity for such applications as semiconductor wafer processing. Shot noise is the statistical noise generated in a photocell by the photon current, and is thus proportional to the square root of the laser power. The shot noise current in a photocell is given by the equation ##EQU1## where q is the charge of an electron, P is the power of the laser, A is the conversion efficiency of the photocell (in amperes per watt), and BW is the bandwidth of detection.
Because shot noise is typically much higher in power than amplifier noise, shot noise limits the sensitivity of the best bright field sensors to detection of particles. In the state of the art, bright field sensors have a sensitivity of about 1 .mu.m.
The dark and bright field sensors in the prior art do not distinguish between spherical and non-spherical particles. In many applications, the particles of interest are non-spherical. For example, most of the particles responsible for contamination in a processing chamber for integrated circuit manufacture are non-spherical (e.g. flakes from the chamber walls). However, the density of such non-spherical particles may be lower than that of spherical particles generated by homogenous nucleation in the plasma stream.
These spherical particles do not represent a source of contamination because either they form in the plasma gas stream downstream from the wafer being manufactured, or they form above the wafer but are suspended above the wafer by electric fields. Without a sensor that is insensitive to spherical particles, the high density of spherical particles produced by homogenous nucleation in the plasma gas stream may mask the relatively low density of non-spherical particles, thereby failing to detect particles which cause wafer contamination and lower manufacturing yield.